1. Field of the Invention
This invention relates generally to means and methods for attaching conducting tabs or external terminals to air depolarized electrode assemblies and, more specifically, to attaching an electrical lead to the cathode assembly of a zinc air cell.
2. Description of the Prior Art
The formation of electrode assemblies with conducting tabs is old in the battery art. A typical example of the method of fabricating or attaching a tab to an electrode or battery plate is shown in the Berg Pat. No. 2,861,115 which is assigned to the same assignee as the present invention. Berg shows a battery plate for an alkaline storage battery in which the electrode comprises nickel powder or plaque surrounding a nickel coated screen or grid. An electrical tab is connected to the battery plate by compressing or coining a portion of the plate and then forcing the crowns on the conducting tab therethrough. In the final step of the assembly, the tab is welded or fused to the screen to produce a low resistance electrical connection. In the Berg process the tines apparently allow gas to escape thus making it feasible to weld the tabs to the screen. While this process and apparatus work well for alkaline battery electrodes, the process of coining or compressing a portion of the electrode assembly of an air depolarized cell and then fusing a tab to the screen has proved unsatisfactory for the electrode assemblies of air depolarized cells. Typically, the air depolarized electrode comprises a separator on one side which is adjacent to a conducting screen that contains an electrode mixture such as carbon and a catalyst dispersed throughout the screen as well as above the screen. Located on the top of the screen is a layer of hydrophobic material that prevents escape of liquid therethrough but allows air to enter the electrode.
The difficulty in forming a low resistance electrical connection to an electrode assembly for air depolarized cells is that the assembly has a noonconducting separator on one side of the grid and a nonconducting hydrophobic member on the other side of the grid. Thus, the electrode assembly is not susceptible to a coining or compacting proess to form a conducting region for receiving a tab with crowns. Also, because of the insulating layers on the outside of the electrode assembly, the fusing operation cannot produce a low resistance electrical connection. Thus, to weld the tab to the crown, the separator material or hydrophobic member must be removed or stripped away if an electrical connection is to be formed between the screen and the tab. Also, because the separator and hydrophobic member are somewhat resilient, it is difficult to force the tines of the crown through either the separator or the hydrophobic member. That is, unless certain precautions are taken, the tines will bend or push the separator against the collecting screen thus providing an inadequate electrical connection.
Thus, to date, in order to form a low resistance electrical connection with the electrode comprised of a separator and carbon catalyst, dispersed around the electrode, it is necessary to take certain precautions. First, the separator and the hydrophobic member must be stripped or cut away in the region where the connection is to be made. Next, the electrode material adjacent and on the current conducting screen must be thoroughly cleaned. Then one can weld the tab to the screen. In addition, if welding or fusing is used, it should be done in the presence of an inert gas such as helium to prevent any carbon on the electrode assembly from burning during the welding operation.
Still another method for forming a low resistance electrical connection is to cut away a portion of the separator, the electrode material and the hydrophobic membrane and attach the tab by mechanically connecting the tab to the screen. However, both welding and fusing in this manner are very time consuming and sometimes difficult to accomplish, thus precluding mass production of these units. Therefore, although possible to form low resistance electrical connections, it is both difficult and time consuming to fasten a connector to the current collecting screen of an electrode assembly.
In still another prior art process, a portion of the screen is extended so it projects beyond the electrode assembly to be used for fastening a tab thereto. However, even if the screen extends beyond the electrode assembly, the normal handling during manufacture of the electrode assembly requires one to clean the screen before one can form a low resistance electrical connection thereto.
In still another embodiment, a portion of the screen is used as the external connector by allowing it to extend beyond the cell.
In still another method of forming a low resistance electrical connection, an electrode pin is inserted through the screen and through the electrode assembly to produce a low resistance electrical connection between the screen and the pin. This technique is described in the co-pending application of Arthur M. Jaggard, Ser. No. 229,274, filed Feb. 25, 1972, titled "Assemblying Electrodes in Electrochemical Cells", which is assigned to the same assignee as the present invention.
Thus, most prior art techniques for providing a connection to the electrode assembly of a cathode of a zinc air cell have been particularly cumbersome and difficult to achieve because of the requirement and necessity of cleaning the surface of the conducting screen before forming a connection to the electrode.
The present invention eliminates the prior art problems by utilization of a tab having crowns thereon which contain cutting edges for piercing the separator, current collection screen and the hydrophobic member. The electrode assembly can thus be fastened to the conductor by positioning the electrode assembly above a tab with a crown and forcing the tines or prongs of the crowns through the entire electrode assembly.
It has been discovered that this process, without any further application of fusing, cleaning, etc., produces a low resistance electrical connection between the screen and the tab. In addition, the use of more than one set of crowns provides torsional resistance to the tab thus making it sturdier for handling in the unassembled state.